Alkanolamine process units remove H.sub.2 S and CO.sub.2 from gaseous process streams, typically by countercurrently contacting an aqueous solution containing from about 20% to about 50% by weight of an alkanolamine with a gas stream containing H.sub.2 S and/or CO.sub.2. For the purpose of this application, it is understood that the terms "alkanolamine" and "ethanolamine" are generic terms including, but not limited to, monoethanolamine, diethanolamine, triethanolamine, and methyl diethanolamine.
The removal of hydrogen sulfide from gaseous streams, such as the waste gases liberated in the course of various chemical and industrial processes, for example, in wood pulping, natural gas and crude oil production and in petroleum refining, has become increasingly important in combating atmospheric pollution. Hydrogen sulfide containing gases not only have an offensive odor, but such gases may cause damage to vegetation, painted surfaces and wildlife, and further may constitute a significant health hazard to humans. Government-wide regulations have increasingly imposed lower tolerances on the content of hydrogen sulfide which can be vented to the atmosphere, and it is now imperative in many localities to remove virtually all the hydrogen sulfide under the penalty of an absolute ban on continuing operation of a plant or the like which produces the hydrogen sulfide-containing gaseous stream. Solutions of water and one or more the alkanolamines are widely used in industry to remove hydrogen sulfide and carbon dioxide from such gaseous streams.
Corrosion in alkanolamine units significantly increases both operating and maintenance costs. The mechanisms of corrosive attack include general corrosive thinning, corrosion-erosion, and stress-corrosion cracking. Corrosion control techniques include the use of more expensive corrosion and erosion resistant alloys, continuous or periodic removal of corrosion-promoting agents in suspended solids by filtration, activated carbon adsorption, or by the addition of corrosion inhibitors. (See Kohl, A. L. and Reisenfeld, F. C., Gas Purification, Gulf Publishing Company, Houston, 1979, pp. 91-105, as well as K. F. Butwell, D. J. Kubec and P. W. Sigmund, "Alkanolamine Treating", Hydrocarbon Processing, March, 1982.)
Further, it has been found that the acid gas sorption capacity in a circulating alkanolamine-water system decreases with time on stream in the absence of added makeup alkanolamine. This performance degradation has been found to be partially attributable to the accumulation of heat stable salts. U.S. Pat. No. 4,795,565 to Yan describes a process for removing heat stable salts from an ethanolamine system by the use of ion exchange resins. The disclosure of U.S. Pat. No. 4,795,565 to Yan is incorporated herein by reference for the operating details both of an ethanolamine acid gas sorption system as well as for the heat stable salt removal process.
Heat stable salts may also be removed from an alkanolamine system by distillation. However, such separation has been limited in the past to relatively mild conditions of temperature and pressure to avoid thermal degradation of the alkanolamine. For example, diethanolamine (DEA) boils at 268.degree. C. at 760 mm Hg pressure and tends to oxidize and decompose at high temperature. For this reason, vacuum distillation has been used to separate heat stable salts from alkanolamine solution.
The chemistry of alkanolamine degradation is discussed in the Butwell et al. article cited above. Briefly, the Butwell et al. article notes that monoethanolamine (MEA) irreversibly degrades to N-(2-hydroxyethyl) ethylene diamine (HEED). HEED shows reduced acid gas removal properties and becomes corrosive at concentrations of at least about 0.4% by weight.
Diglycolamine (DGA), on the other hand, is said to produce a degradation product upon reaction with CO.sub.2 which exhibits different properties. DGA is a registered trademark of Texaco, Inc. which identifies an amine having the chemical formula NH.sub.2 --C.sub.2 H.sub.4 --O--C.sub.2 H.sub.4 --OH. DGA degrades in the presence of CO.sub.2 to form N,N'-bis(hydroxyethoxyethyl) urea (BHEEU) which is similar to HEED in corrosivity but differs in that BHEEU has no acid gas removal properties.
Diethanolamine (DEA) reacts with CO.sub.2 to form N,N'-di(2-hydroxyethyl) piperazine. Unlike HEED and BHEEU, the piperazine compound is noncorrosive and has acid gas removal properties essentially equal to its parent, DEA. See the Butwell et al. article at page 113.
Diisopropylamine (DIPA) readily degrades in the contact with CO.sub.2 to form 3-(2-hydroxypropyl) 5-methyl oxazolidone which shows essentially no acid gas removal properties. See the Butwell et al. article at page 113.
U.S. Pat. No. 4,281,200 to Snoble teaches a process for recovering diisopropanolamine from the cyclic reaction products formed by reacting CO.sub.2 with diisopropanolamine which process comprises reacting the cyclic product with an inorganic base at temperatures between about 105.degree. and 200.degree. C.
Numerous degradation products formed by the reaction of H.sub.2 S, or a mixture of H.sub.2 S and CO.sub.2 with diethanolamine have been reported from analyses of operating diethanolamine acid gas sorption processes and are shown below in Table 1.
TABLE 1 __________________________________________________________________________ COMPOUNDS RESULTING FROM DEA DEGRADATION Name Abbreviation Structural formula __________________________________________________________________________ N,N-Bis (2-hydroxyethyl) piperazine HEP ##STR1## N,N,N-tris (2-hydroxyethyl) ethylenediamine THEED ##STR2## Hydroxyethyl imidazolidone HEI ##STR3## N-Methyldiethanolamine MDEA ##STR4## Oxazolidone OZO ##STR5## Aminoethykethanolamine AEEA ##STR6## Bis-(2-hydroxy ethyl) glycine BHG ##STR7## __________________________________________________________________________
Accumulation of these and other degradation products in the alkanolamine system reduces acid gas sorption capacity and increases the corrosivity of the alkanolamine solution.